Deaeration of feed materials in an extrusion process

ABSTRACT

In a method for molding a raw material including a plastic material or a mixture of plastic and non-plastic materials fed into a cylinder, the said raw material is subjected to a melt-kneading performed under oxygen-free conditions, and at the same time the molten material is sent from the tip of an extrusion screw towards the head section of the cylinder in straight flow conditions. In the method satisfactory molding can be made for plastics of poor fluidity or thermal stability or for plastic materials mixed with more than 10 to 30% by weight of wood flour, paper powders, stone powders, metal powders, fiber reinforcements and the like, without causing molding defects such as burn or cavitation; and also for raw materials mixed with more than several per cent by weight of metal powders or stone powders without causing excessive wear at the extrusion screw.

TECHNICAL FIELD

The present invention relates to a method for molding plastics and othermaterials. More specifically, the present invention relates to a moldingmethod suitable for application in molding materials having poorfluidity or thermal stability, which typically can include plasticmaterials such as polyethylene and others having ultra-high molecularweight, or mixtures of plastic materials such as polyvinyl chloride andnon-plastic materials such as wood flour, paper powders, stone powders,metal powders, fiber reinforcements, and the like.

BACKGROUND ART

In the field of technologies for molding plastics, numerous improvementshave been proposed and implemented to bring about remarkable progress inthe related area.

In Japanese Patent Application Laid-Open No. 190891/1994, the presentinventor has proposed a molding technique for plastic materials whereinthe production rate and quality of the molded products can be improvedby increasing bulk density of the raw material within the extrusioncylinder.

On the other hand, in line with requirements related to diversificationand recycled use of plastic moldings in recent years, new products haveemerged and are used in applications such as building materials,examples of which include molded products of wood-like appearance usingraw materials comprising mixtures of plastic materials and wood flour,or molded products of marble-like appearance using raw materialscomprising mixtures of plastic materials and marble stone powders.

In the prior art, however, molded products having wood-like appearancecomprising mixtures of plastic materials and wood flour, or thosecomprising mixtures of plastic materials and marble stone powders lackedthe appearance and tactility of genuine wood or marble and thereforewere unable to offer higher commercial values because the mixing ratiosof wood flour or stone powders to the plastic materials were limitedtypically to a level less than 20 to 30% by weight.

The reason that the mixing ratios of wood flour or stone powders must belimited to a level less than 20 to 30% by weight as above is that, rawmaterials comprising plastic materials mixed with 20 to 30% by weight ormore of non-plastic materials such as wood flour or stone powderstypically exhibit reduced fluidity and thermal stability when anextrusion molding is performed on them. This in turn causes anoverheating of the non-plastic materials contained in the raw material,leading to carbonization and a phenomenon so-called “burn” in the moldedproducts. This phenomenon can occur in a similar fashion on rawmaterials comprising plastic materials mixed with more than 30% byweight of paper powders.

Meanwhile, in the case of raw materials having poor fluidity or thermalstability, the molded products sometimes develop cavities within them(i.e. “cavitation”), even though there were no burn-related problems.

Such phenomena as burn or cavitation above have also been experiencedwhen a plastic material of poor fluidity or thermal stability is usedalone.

Furthermore, when a molding operation is performed on materials mixedwith 5% by weight or more of stone powders or those mixed with more thanseveral percent by weight of metal powders using a conventionalextruding machine, wear on the extrusion screw can become excessive,particularly at its tip portion, requiring a frequent replacement of thescrew at an increased cost of operation.

As a result of extensive research conducted with a view to eliminatingaforementioned problems, the present inventor has found that a smoothand high quality molding operation can be made available for plasticmaterials of poor fluidity or thermal stability, or for raw materialsbased on plastic materials mixed with non-plastic materials such as woodflour, paper powders, stone powders, metal powders, or fiberreinforcements at ratios higher than that conventionally applied, bycarrying out melt-kneading of raw materials under oxygen-freeconditions, shortening melt-kneading time duration, feeding rawmaterials from the screw tip in straight flow conditions, and/orreducing the shearing force working at the screw tip material.

The present invention has been made in this situation and has an objectof providing a method for performing satisfactory molding for plasticshaving poor fluidity or thermal stability or for plastic materials mixedwith more than 10 to 30% by weight of wood flour, paper powders, stonepowders, metal powders, fiber reinforcements, and the like, withoutcausing molding defects such as burn or cavitation, and also for rawmaterials mixed with more than several percent by weight of metalpowders or stone powders without causing excessive wear at the extrusionscrew.

DISCLOSURE OF THE INVENTION

In accordance with the method for molding plastics and other materialsof the present invention, a raw material comprising a plastic materialor a mixture of plastic and non-plastic materials is fed into a cylinderwherein said raw material is subjected to a melt-kneading performedunder oxygen-free conditions.

One example of the possible methods for providing oxygen-free conditionsfor a raw material comprises feeding the raw material from a hopper intoa cylinder through a valve, a connecting tube, and an opening to thecylinder while the raw material is subjected to deaeration as it travelsthrough the path from said valve to said cylinder and, at the same time,effecting an additional deaeration in the vicinity of the opening tosaid cylinder with a suction force equal to or greater than that appliedfor the deaeration provided in the path from said valve to saidcylinder, thereby achieving the desired oxygen-free conditions for theraw material. It is more desirable if the above hopper is hermeticallysealed and, furthermore, feeding of raw materials to said hopper isprovided alternately from a plurality of sub-hoppers that are alsohermetically sealed.

Since the above-described method helps hinder oxidative phenomenaoccurring within the cylinder, carbonization of the raw material can beminimized and therefore the burn defects in the molded products can beavoided.

Furthermore, in the method for molding plastics and other materialsaccording to the present invention, it is desirable to feed a molten rawmaterial from the tip of the screw to the head section of the cylinderwhile maintaining straight flow conditions. As a specific means to feeda raw material to the head section of the cylinder in straight flowconditions, use of a screw having a tip angle of from 30 to 120 degreescan be given, by way of an example.

An arrangement for the screw tip angle such as described above enablesto eliminate burn defects occurring due to a part of the molten rawmaterial being trapped in the swirl generated around the screw tip andflowing stagnantly, or cavitation defects caused due to starvation ofthe molten raw material occurring otherwise in the area around thecenter of the screw tip.

Moreover, to feed a molten resin to the head section of the cylinder inbetter straight flow conditions, it is desirable to reduce the shearingforce working on the molten resin.

In the present specification, the passage “oxygen-free conditions for araw material” refers to the conditions wherein gaseous substances suchas air are removed from the raw material itself as well as from theinterspaces held in the raw material bulk, thereby leaving only a smallamount of oxygen remaining in it.

Also in the present specification, the word “molten” can sometimes referto a semi-molten state in addition to a completely molten state.

Furthermore, in the present specification, the passage “straight flowconditions” (or so-called “plug flow”) is defined as conditions whereina molten raw material flows along the screw tip with an approximatelyequal flow velocity across the radial sections and without involvingirregular flows such as a turbulent flow.

In the above-mentioned method for molding plastics and other materials,raw materials containing from 10 to 80% by weight of non-plasticmaterials are also included as the object of molding, as well as rawmaterials comprising plastic materials in either pellet or powder formand non-plastic, powder form materials, both being fed from hoppers.

The equipment for molding plastics and other materials according to thepresent invention is provided with a pair of hoppers and a main hoppermounted underneath them, a sealing valve connected to said hopper, aconnecting tube placed between said sealing valve and an opening of acylinder, primary suction holes provided on said connecting tube, ascrew inserted in said cylinder, and secondary suction holes provided inthe vicinity of the opening at the inner diameter wall of said cylinder,with a ratio of the cylinder length to the screw length being from1:0.98 to 1:0.75.

In the above arrangement, it is more desirable to configure a pluralityof sub-hoppers on top of the above-mentioned hopper, each beingconnected through a sealing valve.

A configuration such as described above enables feeding of raw materialsinto the hopper in a continuous mode while avoiding entraining air.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side sectional view of a molding machine in oneembodiment of the present invention.

FIGS. 2(a) to 2(c) are schematic diagrams illustrating the manner inwhich the secondary suction holes are formed in one embodiment of thepresent invention.

FIGS. 3(a) and 3(b) are schematic diagrams illustrating the conditionsof raw material being trapped and stagnant and also in a turbulent flowoccurring when a screw having a tip angle greater than 120 degrees isused, whereas

FIG. 3(c) is a schematic diagram showing the raw material condition in astraight flow occurring when a screw having a tip angle smaller than 120degrees is used.

FIG. 4 shows an enlarged side sectional view of the sealing section of amolding machine according to the present invention.

FIG. 5 shows a side view section of the cylinder section of a moldingmachine in the prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

The mode for carrying out the present invention will be explained inmore detail below.

First, a molding machine in one embodiment of the present invention isdescribed referring to the drawings presented.

FIG. 1 shows a sectional view of the configuration of a molding machinein one embodiment of the present invention.

The equipment shown in the drawing has a hopper 10, a rotaryvalve(sealing valve, valve) 20, a connecting tube 30, a cylinder 40, anda screw 50.

The hopper 10 is equipped with two sub-hoppers 11 a and 11 b on its top,and the sub-hoppers 11 a and 11 b are connected to a raw material feeder13 through ball valves 12 a and 12 b while at the same time connected tothe hopper 10 via ball valves 14 a and 14 b.

Moreover, a suction tube 15 is connected to an upper part of the hopper10 to evacuate air from the hopper 10.

When the raw material is fed from one of the sub-hoppers, 11 a, into thehopper 10 in the above arrangement, the ball valve 12 a located betweenthe sub-hopper 11 a and the raw material feeder 13 is closed, while atthe same time the ball valve 14 a located before the hopper 10 isopened. Concurrently, the ball valve 14 b located between the othersub-hopper 11 b and the hopper 10 is closed, while at the same time theball valve 12 b located after the raw material feeder 13 is opened toreceive the raw material into the sub-hopper 11 b.

The above-mentioned arrangement enables to limit the entrance of airinto the hopper 10 at the time of receiving the raw material to aminimum, while maintaining the level of the raw material inside thehopper 10 roughly constant.

The hopper 10 meanwhile is connected to an opening 41 of the cylinder 40through the rotary valve 20 and the connecting tube 30 attachedunderneath the rotary valve.

In the above arrangement, the rotary valve 20 is designed to feed theraw material from the hopper 10 to the connecting tube 30 by rotating avalve unit 22 with a driving motor 21, sending a constant quantity witheach rotating movement, and also to prevent air remaining in the hopper10 from flowing into the connecting tube 30 with its sealed structure.

The connecting tube 30 is furnished with primary suction holes 31 toremove air from the raw material flowing inside the connecting tube 30.The primary suction holes 31 are provided at all or a part of the innercircumference of the connecting tube 30 into a sunken shape, with theapertures furnished with a filter element 32.

The primary suction holes 31 are then connected through one or more ofpiping 33 (only one is shown in the drawing) to a suction pump not shownin the drawing.

The filter element 32, not shown in the drawing, is preferablyconfigured with one to three mesh screens placed on a holder made of ametal plate having punched perforations. A desirable aperture rate forthe mesh screen holder to be used in the filter element 32 is from 60%to 90%.

Also, a desirable pore size of the mesh screens to be used in the filterelement 32 is from 60 mesh to 100 mesh.

With the above-mentioned configuration, the amount of the raw materialescaping through the suction holes together with air can be restrictedto a minimum, while allowing efficient deaeration of the raw material.

The cylinder 40 has the screw 50 inserted therein, together with abreaker plate 60 (i.e. die) attached at its head.

At the bottom and/or near the bottom of the cylinder 40 facing theopening 41 are provided secondary suction holes 42 for deaeration of theraw material fed into the cylinder 40, and also for drawing andreversing the heat within the cylinder 40 back to the vicinity of theopening 41.

The location of the secondary suction holes 42 is not specificallyrestricted, as long as the raw materials are in the form of pellets witha relatively high bulk density. However, when the raw materials with alow bulk density, such as of a powder form, are used, it is preferableto locate the secondary suction holes 42 within 30 to 60 degrees on bothsides of the lowest position of the cylinder 40, i.e. within acircumferential range of 60 to 120 degrees at the bottom section of thecylinder 40.

More specifically, possible arrangements for the secondary suction holes42 include a hollow shaped suction hole 42 formed across the 120-degreecircumferential range at the bottom section as shown in FIG. 2(a), asuction hole 42 formed only at the bottom location as shown in FIG.2(b), and a plurality of suction holes 42 formed within acircumferential range of 60 to 120 degrees at the bottom section of thecylinder 40, as shown in FIG. 2(c).

In the above arrangement, a filter element 43 may be provided at theaperture of the suction hole 42 as shown in FIG. 2(a), or varieddiameters may be used when a plurality of the suction holes 42 is formedas shown in FIG. 2(c).

Additionally, the secondary suction holes 42 are connected throughpiping 44 to a suction pump not shown in the drawing.

Performing deaeration through the secondary suction holes 42 provided atpositions within the 120-degree circumferential range at the bottomsection of the cylinder 40 as described above allows the raw materialfed into the cylinder 40 to be attracted towards the bottom of thecylinder 40. This in turn enables a smooth supply of the raw materialinto the cylinder 40.

By contrast, if the secondary suction holes 42 are provided at positionsoutside of the 120-degree circumferential range at the bottom section ofthe cylinder 40, the raw material fed from the hopper into the cylinder40 can be attracted to the inner diameter wall of the cylinder 40 nearthe opening 41 and accumulated nearby. This can interfere with thesubsequent supply of the raw material into the cylinder 40, causingfluctuation or shortage of the feedstock supply and thereby leading to alower productivity and inferior quality.

The screw 50 is rotated by a drive motor housed inside a drive unit 51but not shown in the drawing, and works to melt and knead the rawmaterial fed into the cylinder 40. In addition, the extruder of thepresent embodiment has a sealing section 43 to prevent the entrance ofair from the driving unit 51 into the cylinder 40.

Next, the sealing section 43 of the extruder of the present embodimentis described in detail.

FIG. 4 shows a cross-sectional view of the sealing section 43. Asillustrated in the drawing, with the sealing section 43 of the presentembodiment, an oil seal 431 is provided via a sliding shoe 433 at theouter circumference of a sleeve 432 for power transmission to which theroot 50 a of the screw 50 is inserted. The oil seal 431 is not placeddirectly on the outer circumference of the screw root 50 a. The oil seal431 is positioned closer to the driving unit 51 than the secondarysuction holes 42 so that air does not flow from the driving unit 51 tothe secondary suction holes 42.

The above arrangement allows an appropriate space 434 to be formedbetween the screw root 50 a and the oil seal 431.

Locating the oil seal 431 at a position away from the screw root 50 a asdescribed above eliminates the chances for potential damages caused onthe oil seal 431 by the raw material when the screw 50 is withdrawn fromor inserted into the cylinder 40 for maintenance or other reasons. Inother words, chances of such a damage arise when the screw 50 iswithdrawn from the cylinder 40 allowing any raw material remaining inthe connecting tube 30 to fall into the cylinder 40.

If the screw 50 is inserted back into the cylinder 40 in the abovecondition, the raw material inside the cylinder 40 is pushed by thescrew root 50 a towards the driving unit 51, thereby damaging an oilseal if this is positioned directly on the inner wall of the cylinder40. A damaged oil seal inevitably allows an inflow of air from thedriving unit 51 into the cylinder 40 to reduce the effectiveness ofdeaeration performed through the secondary suction holes 42.

However, with presence of the space 434 provided between the inner wallof the cylinder 40 and the oil seal 431 as in the sealing section 43 ofthe present embodiment, raw material that has fallen from the connectingtube 30 into the cylinder 40 is collected in the space 434 and isprevented from entering between the oil seal 431 and the sliding shoe433. Consequently, the oil seal 431 can be kept intact and free from thedamages by the raw material that have been experienced with aconventional machine when the screw is drawn from or inserted into thecylinder for maintenance or other reasons.

Note here that any raw material trapped in the space 434 is removed whenthe sealing section 43 is dismantled for withdrawing the screw 50, andtherefore will not accumulate in the space 434.

Meanwhile, the screw 50 is designed shorter than the cylinder 40 toprovide a space 55 in front of the tip of the screw 50. Specifically,the screw length is set at a ratio from 0.98 to 0.75 to 1.00 of thecylinder length, depending on products to be molded, to provide thespace 55 on the breaker plate 60 (die) side of the cylinder 40 withapproximately 2-25% of its total length.

The space 55 acts to reduce the shearing force working on the molten andkneaded raw material immediately before extrusion from the cylinder 40so that unnecessary heating of the raw material can be prevented, aswell as to reduce unevenness in the flow of the molten material (i.e. toimprove the regularity of the straight flow), while lowering thematerial temperature to adjust the viscosity of the material to a leveldesired for the molding operation. This prevents oxidation whichotherwise occurs when the materials are extruded under high temperatureconditions, thereby ensuring production of high quality molded articles.

The size of the above-mentioned space 55 therefore is determined withinthe range specified in the above and according to the raw material to bemolded, so that excessive frictional heat or uneven kneading is notcaused by too narrow space 55 or, conversely, too much space will notmake the kneading strength insufficient.

With regard to the tip section 52 of the screw 50 (or the tip of a screwcap if one is used), the tip angle α as represented in FIG. 3 isselected from an approximate range from 30 to 120 degrees depending onthe raw material to be molded. Setting the tip angle for the screw 50within an approximate range from 30 to 120 degrees ensures feeding ofthe raw material from the tip of the screw 50 towards the breaker plate60 (die) in a straight flow.

The above consideration is required from the fact that there are widedifferences in the fluidity of plastic materials or that of compositematerials comprising plastic and non-plastic materials.

For instance, when molding a standard raw material having a lowviscosity and relatively good fluidity using a machine equipped with ascrew in which the tip angle α is the standard 160 degrees, while theflow of the raw material becomes somewhat slow around the central axisin front of the screw tip, the low viscosity of the material helps keepthe flow from becoming stagnant, enabling a satisfactory molding eventhough there may be a slight tendency of oxidation (see illustration inFIG. 3(a)).

However, when molding a plastic material or a mixture of plastic andnon-plastic materials having an unsatisfactory fluidity with a machineusing a screw 50 with the tip angle α of 160 degrees, the flow of theraw material around the central axis in front of the screw tip becomesstagnant, causing an oxidative burn or cavitation as a result.Consequently, it becomes impossible to perform a normal moldingoperation (see illustration in FIG. 3(b)).

On the other hand, even when the molding operation is provided for aplastic material or a mixture of plastic and non-plastic materialshaving an unsatisfactory fluidity, selecting the tip angle α of thescrew 50 at about 120 degrees or less ensures the feeding of the rawmaterial under straight flow conditions (so-called plug flow), as thematerial is guided along the tip of the screw 50, thereby enabling toalmost completely eliminate the undesirable phenomena mentioned in theabove.

It is noted here that, while selecting a tip section 52 of the screw 50with an angle smaller than 30 degrees might increase the above-mentionedeffect, it could make the size of the spade 55 unnecessarily large.

The specific value for the tip angle α of the screw 50 is determinedaccordingly with the material and conditions for the intended moldingoperation.

Next, one embodiment of the method for molding plastics and othermaterials according to the present invention is described in detailreferring to FIG. 1.

The raw material comprising a plastic material or a mixture of plasticand non-plastic materials is supplied from the raw material feeder 13 tothe hopper 10 via two sub-hoppers 11 a and 11 b. In the presentembodiment, the raw material is fed alternately from the sub-hoppers 11a and 11 b by controlling and switching the ball valves 12 a and 12 b,and ball valves 14 a and 14 b, for the respective sub-hoppers.

The above-mentioned method of supplying the raw material helps minimizethe entrance of air into the hopper 10, while enabling a continuousfeeding of the material. Consequently, the quantity of the raw materialand the vacuum inside the hopper 10 are maintained roughly at constantlevels.

In addition to the above, air inside the hopper 10 is removed throughthe suction tube 15 connected to the hopper 10.

The raw material in the hopper 10 is sent to the connecting tube 30 bythe rotation of the valve unit 22 inside the rotary valve 20, a constantquantity being sent with each rotating movement, and then to the opening41 of the cylinder 40 through the connecting tube 30. Meanwhile,moisture or gaseous substances such as air are removed from the rawmaterial as they are drawn through primary suction holes 31 provided atthe connecting tube 30, while the raw material falls through the pathfrom the outlet of the rotary valve 20 via the connecting tube 30 to theopening 41 of the cylinder 40.

In the above arrangement, in the section where the hopper 10 connects tothe rotary valve 20, the raw material is attracted from the former tothe latter because of the suction effect from the primary and secondarysuction holes 31 and 42. Consequently, a constant quantity of the rawmaterial is sent into the pocket 22 a of the valve unit 22 regardless ofthe quantity of the raw material remaining in the hopper 10, therebyperforming a certain measuring function.

In this event, it is obvious that if the quantity of the raw materialremaining in the hopper 10 is maintained roughly at a constant level,the quantity of the raw material falling from the hopper 10 to thepocket 22 a becomes more consistent to enable a more accurate supply ofthe raw material to the cylinder.

The suction force at the primary suction holes 31 is preferably set from400 mmHg to 700 mmHg, depending on the type of raw materials. Settingthe suction force within the above range enables satisfactory deaerationof the raw material while preventing the raw material from escapingthrough the filter element 32.

In addition, it is desirable to set the suction force at the primarysuction holes 31 at a level equal to or higher than the suction force atthe suction tube 15 provided on the hopper 10 and, at the same time, ata level lower than that at the secondary suction holes 42, for whichdetailed explanation will be given later.

The above setting is required because if the suction force at theprimary suction holes 31 is weaker than that at the suction tube 15, itcan hinder the smooth supply of the raw material from the hopper 10 tothe connecting tube 30. By the same token, if the suction force at theprimary suction holes 31 is stronger than that at the secondary suctionholes 42, it can attract the raw material towards the area of theprimary suction holes 31 to prevent a smooth flow into the cylinder 40,and eventually to clog the connecting tube 30 with the raw materialstaying inside.

It is noted here that, although there is a slight inflow of air at therotary valve 20 from the bearing section that is not shown in thedrawing, it does not adversely affect the arrangement as the leakage isnegligibly small compared with the degree of deaeration provided. On thecontrary, the inflow of air from the bearing into the rotary valve 20acts to prevent ingress of the raw material into the bearing, therebyeliminating the so-called bite of the raw material at the bearing toensure smooth rotations of the bearing and therefore a steady feed ofthe material by the rotary valve 20.

The raw material thus deaerated as described above is then fed from theopening 41 into the cylinder 40 wherein another suction is provided atthe secondary suction holes 42. As described previously, the suctionforce at the secondary suction holes 42 is required to be stronger thanthat of the primary suction holes 31 and, more specifically, to be setbetween 500 mmHg and 760 mmHg depending on the material.

The above suction is provided at the bottom side of the cylinder 40opposing the opening 41, more specifically, within a circumferentialrange of roughly 60 to 120 degrees at the bottom section of the cylinder40 to amass the raw material inside the cylinder 40 around this area.

Setting the suction force and the position for the secondary suctionholes 42 as described above enables a stable and ample supply of the rawmaterial into the cylinder 40. Also, by setting the suction forces atthe primary suction holes 31 and the secondary suction holes 42 as wellas the pressure difference between them in the manner as describedabove, the raw material is prevented from flowing out to outside of thecylinder 40 together with air as deaeration is performed at thesecondary suction holes 42.

The application of suction force at the secondary suction holes 42provides the raw material inside the cylinder 40 with further deaerationto increase its bulk density, while at the same time bringing the systemto oxygen-free conditions.

In the above arrangement, an improved deaeration effect can be obtainedif sub-hoppers 11 a and 11 b as shown in FIG. 1 are installed on top ofthe hopper 10 to place the hopper 10 under a sealed condition, or thelength of the connecting tube 30 between the hopper 10 and the openingof the cylinder 40 is increased. In these arrangements, it is likelythat the raw material will cause a clogging at the hopper 10 and/or inthe connecting tube 30 if a suction force is not applied at least fromthe secondary suction holes 42. In other words, the above-mentionedtechniques can be considered effective when the suction force is appliedfrom the secondary suction holes 42.

Accordingly, while measures such as arranging the sub-hoppers 11 a and11 b, or making the connecting tube 30 longer can help improve theeffectiveness of the deaeration, they can be dispensed with depending onthe molding conditions, types of raw materials, and other factors.

In an additional function of the present embodiment, the suction fromthe secondary suction holes 42 causes a part of the heat within thecylinder 40 to flow back to the vicinity of the opening 41 of thecylinder 40, to expedite melting of the raw material fed from theopening 41 into the cylinder 40.

Furthermore, the suction from the secondary suction holes 42 causes apart of the raw material within the cylinder 40 to be drawn back toincrease the bulk density of the material in the vicinity of the opening41 of the cylinder 40. When such a raw material having higher bulkdensity is pushed by the screw 50 from nearby the opening 41 towards thefront side of the cylinder 40 a strong shearing force works on thedenser material to generate frictional heat.

Thus, the suction provided at the secondary suction holes 42 acts tostart heating of the raw material having higher bulk density in thevicinity of the opening 41 of the cylinder 40 with the heat flowing backfrom the forward section of the cylinder 40 as well as with thefrictional heat generated by the increased bulk density of the rawmaterial.

Consequently, with the aforementioned method, a sufficient degree ofpre-heating can be provided to the raw material in the vicinity of theopening of the cylinder 40 or in the first zone C1 as shown in FIG. 1 toinitiate melting of the material at a relatively early stage.

The raw material for which heating and/or melting actions are initiatedin the first zone C1 in the cylinder 40 is sent to the second zone C2,where the material is almost completely melted with a furthercompression, and thereafter is given a sufficient amount of kneading inthe third zone C3 to homogenize the material.

Since the raw material is heated under oxygen-free conditions throughoutthe above process, the raw material receives little oxidative effect andtherefore scarcely present defects such as a “burn”.

Moreover, the increased bulk density in the vicinity of the opening 41due to the suction provided at the secondary suction holes 42 and theresultant frictional heat generated by the shearing force, together withthe heat flowing back from the forward section of the cylinder 40,enables a heating of the raw material to be started immediately and at ahigher temperature.

Consequently, there is no need to furnish the molding machine with along pre-heating zone as in the prior art, thereby enabling a shorterover-all length of the molding machine (a reduction by 20% or so ispossible), and a shorter time duration for the raw material to beexposed under heat.

As described in the above, since the melting and kneading can beperformed quickly with a shorter molding machine (i.e. with a shorterprocessing distance), it enables a screw designed with a shorter lengthand also a space to be provided in the head section of a moldingmachine, making it possible to prevent thermal degradation of the rawmaterial as well as wear on the screw tip.

While the degree of wear on the screw is most severe at its tip where itreceives the largest frictional force, since the method for molding inthe present invention provides for the raw material feeding from thescrew tip toward the cylinder head section to be performed in straightflow conditions and also with a reduced shearing force, it reducesfriction with the raw material and, accordingly, reduces wear at thescrew tip material as well.

The raw material thus thoroughly melt and kneaded as described above isthen sent to the fourth zone C4 in front of the screw tip, where theshearing force to the material is reduced and the temperature islowered, before it is extruded out from the breaker plate 60 (die).

In the above arrangement, since the angle α of the tip section 52 of thescrew 50 is set within a range between 30 degrees and 120 degrees, theraw material is fed towards the breaker plate 60 (die) in straight flowconditions without being trapped in a swirl generated around the screwtip or causing a turbulent flow, as shown in FIG. 3(c).

The molding methods described in the foregoing can be satisfactorilyimplemented in applications with a raw material containing as much as 30to 80% by weight of powder form, non-plastic materials in it, not tomention those with less than 30% of such materials. Specifically, moldedproducts with satisfactory quality have been obtained with a non-plasticmaterial content at 30 to 80% by weight in the case of wood flour andpaper powders, 20 to 80% by weight for stone powders, and 5 to 80% byweight for metal powders.

It should be noted here that the molding methods and equipment of thepresent invention are not limited to the embodiments presented in theforegoing and that various changes or variations may be made withoutdeparting from the spirit and scope of the invention. For example, thepresent invention can be applied not only to extrusion molding but alsoto other plastic forming processes such as injection molding, or also asa pelletizer to prepare pellets of raw materials. In particular, theability of the present invention to perform a pelletizing operation at alow temperature is an important feature as it makes the postformingextremely easy.

EXAMPLES

In the following, one embodiment example of the present invention iscompared with a prior art comparative example.

[EXAMPLE]

<Molding conditions>

(1) Uniaxial extrusion machine (an extruder with a 65 mm diameter)

Cylinder length (L)/Screw length (D)=25/23.5 Screw rotation at 40 rpm

(2) Suction forces:

Suction at primary suction holes: 500 mmHg

Suction at secondary suction holes: 650 mmHg

(3) Raw materials:

Polyvinyl chloride as plastic material: 50% by weight

Wood flour as non-plastic material: 50% by weight

(4) Process time: 80 hours

<Results>

The heating of the raw material started in the vicinity of the cylinderopening, and a uniform material thoroughly melt-kneaded and having awood-like appearance was extruded without any burn or cavitation.

Extrusion rate: 81 kg/Hr

Evaluation: Smooth surface, no burn, genuine wood-like appearance, goodmoldability

Conventional Example

<Molding conditions>

(1) Uniaxial extrusion machine (an extruder shown in FIG. 4, with a 65mm diameter)

Cylinder length (L)/Screw length (D)=25/25

Screw rotation at 20 rpm

(2) No deaeration was provided on the raw material.

(3) Raw materials:

Polyvinyl chloride as plastic material: 70% by weight

Wood flour as non-plastic material: 30% by weight

(4) Process time: 8 hours

<Results>

In reference to FIG. 5 illustrating a conventional extruder, the rawmaterial was transferred in the section denoted with C1′, compressed inthe C2′ section, and melted in the C3′ and C4′ sections. Burnt materialsstarted to contaminate the product as the process time elapsed tonecessitate stoppage and an overhaul servicing on the molding machine.

Extrusion rate: 25 kg/Hr

Evaluation: The raw material was excessively kneaded and yielded aproduct with cavities in it as well as burn marks on the surface whichwas rough, failing to present a genuine wood-like appearance.

The method for molding plastics and other materials according to thepresent invention enables a molding operation on raw materials havingpoor fluidity or thermal stability to yield material with thoroughlymelt-kneaded and uniform quality to result in exceedingly superiormolded products without burn or cavitation therein, with high efficiencyand within a short processing time. Consequently, the method allowsstable operation of a molding machine for a longer duration tosubstantially increase productivity.

Further, in accordance with the molding equipment of the presentinvention, molding of materials with poor fluidity or thermal stability,which have been regarded as difficult-to-mold materials in the priorart, can be made possible with a simple and compact equipment.

Furthermore, even if the raw material is a plastic material withrelatively good fluidity, when a product having a fine shape such asfilaments for textile or a thin form such as film is molded, air,volatile substance, or moisture contained in the raw material evaporatesand ruptures when the material leaves the die lip, leaving a decomposedcontaminant that is so-called “eye wax” at the die lip. When such acontaminant deposits on the die lip, quality defects such as a roughproduct surface, streaks, transparency failures, or a filamentdiscontinuation develop to eventually lead to a lowered productivity.

According to the present invention, even when products having a fineshape such as filaments for textile or a thin form such as films aremolded with a raw material comprising a plastic material with relativelygood fluidity, by providing the plastic material with melt-kneadingunder oxygen-free conditions within the extruder cylinder, and also byfeeding the molten material from the screw tip to the head section ofsaid cylinder in a straight flow, good quality products can be moldedwith a high productivity while avoiding problems such as “eye wax”described above.

Industrial Applicability

In accordance with the equipment for molding plastics and othermaterials of the present invention, satisfactory molding is madepossible for plastics having poor fluidity or thermal stability, or forraw materials comprising plastic materials mixed with non-plasticmaterials such as wood flour, paper powders, stone powders, metalpowders, fiber reinforcements and the like. The present invention,therefore, can be applied for example to molding of pipes, sheets,filaments, preforms, packaging materials, optical fibers, wire coating,construction materials, automobile interior fittings, exterior andinterior fittings for home electronic appliances, and the like.

What is claimed is:
 1. A method for molding plastics and othermaterials, comprising: supplying a raw material to a hermetically sealedhopper alternately from a plurality of sub-hoppers, providing said rawmaterial in the hopper to a cylinder through an opening of the cylinder,removing air inside the cylinder at an area in the cylinder near theopening thereof through at least one air suction hole of the cylinder tothereby remove air from the raw material and bring the raw material inan oxygen-free condition so that the raw material in the cylinder isdrawn near the opening in the cylinder to increase bulk density of theraw material near the opening, and rotating a screw disposed in thecylinder to provide a shearing force to the raw material with theincreased bulk density to thereby generate frictional heat to melt-kneadthe raw material, said frictional heat being allowed to flow back to thearea near the opening by removal of air so that melting of the rawmaterial is started from the-area near the opening.
 2. The method formolding plastics and other materials according to claim 1, wherein asuction force at the path from said valve to said cylinder is in therange from 400 to 700 mmHg and a suction force within said cylinder isin a range from 500 to 760 mmHg.
 3. The method for molding plastics andother materials according to claim 1, wherein the screw is located inthe cylinder such that a ratio of a length of the cylinder to a lengthof the screw is 1 to 0.98-0.75 to form a space at a tip of the cylinder,said raw material being sent from a tip of said screw towards a headsection of said cylinder under a straight flow condition.
 4. The methodfor molding plastics and other materials according to claim 3, whereinthe shearing force working on a molten raw material is reduced when saidmaterial is sent from the tip of said screw towards the head section ofsaid cylinder.
 5. The method for molding plastics and other materialsaccording to claim 1, wherein the raw material comprises a mixture of 20to 90% by weight of plastic material and 10 to 80% by weight ofnon-plastic material.
 6. The method for molding plastics and othermaterials according to claim 1, wherein the raw material comprisingplastic materials in either pellet or powder form and non-plastic inpowder form materials are fed from the hopper into the cylinder.
 7. Themethod for molding plastics and other materials according to claim 1,wherein said raw material is supplied to the cylinder from the hopperthrough a connecting tube, a valve and the opening, air being initiallyremoved from said raw material at a path from the valve to the cylinderat a pressure equal to or less than that inside the cylinder.
 8. Themethod for molding plastic and other materials according to claim 1,wherein said at least one air suction hole is a hollow shaped suctionhole formed across a predetermined circumferential range and one holecommunicating with the hollow shaped suction hole.
 9. The method formolding plastic and other materials according to claim 8, wherein afilter is situated in the hollow shaped suction hole.
 10. The method formolding plastics and other materials according to claim 1, wherein saidat least one air suction hole includes a plurality of holes formed atthe circumferenctial range of 60 to 120 (degrees..
 11. The method formolding plastics and other materials according to claim 1, wherein saidsub-hoppers are sealed.
 12. The method for molding plastics and othermaterials according to claim 11, wherein the air inside the cylinder isremoved through the at least one air suction hole located within acircumferential range of 60 to 120 degrees at a bottom section of thecylinder.
 13. A method for molding plastic and other materials,comprising: supplying a raw material including a plastic material or amixture of a plastic and non-plastic material to a hermetically sealedhopper alternately from a plurality of sub-hoppers, providing the rawmaterial from the hopper to a cylinder through a path including a valve,a connecting tube and a cylinder opening, removing air from the rawmaterial in the path from the valve to the cylinder, removing airfurther from the raw material at an area in the cylinder near thecylinder opening at a suction pressure at least equal to a pressure forremoving the air in the path from the valve to the cylinder so that theraw material is brought to an oxygen-free condition, and rotating ascrew disposed in the cylinder to provide a shearing force to the rawmaterial to thereby generate frictional heat to melt-knead the rawmaterial, said frictional heat being allowed to flow back to the areanear the opening by removal of air so that the melting of the rawmaterial is started from the area near the opening.
 14. The method formolding plastics and other materials according to claim 13, wherein saidsub-hoppers are sealed.